An ideal circular single-mode fiber supports two modes with orthogonal polarizations and identical propagation constants. In such an ideal fiber, the state of polarization is maintained along its entire length. However, in real single-mode fibers, deviations from the ideal such as elliptic cores, twists or bends, and anisotropic stresses result in a difference in the propagation constants for these two modes. This induced briefringence leads to variations in the state of polarization of the transmitted light along the fiber. The state of polarization also changes slowly with time due to temperature and pressure changes (I. P. Kaminow, "Polarization in Optical Fibers", IEEE J. Quant. Electron., Vol. QE-17, No. 1, Jan. 1981, pp. 15-22).
The unpredictability of the state of polarization presents a potentially severe detection problem in coherent optical communications systems. In order to exploit the potential advantages of these systems the polarization states of the received signal wave and the local oscillator wave must be matched (I. P. Kaminow, "Polarization in Optical Fibers", IEEE J. Quant. Electron., Vol. QE-17, No. 1, Jan. 1981, pp. 15-22; and T. Okoshi, "Recent Advances in Coherent Optical Fiber Communication Systems", J. Lightwave Tech., Vol. LT-5, No. 1, Jan. 1987, pp. 44-52). A mismatch may severely degrade the receiver sensitivity. In particular, when the polarization states are orthogonal, complete fading results.
We can quantify the effects of this mismatch by considering the signal-to-noise ratio (SNR) at the output of the IF filter in a typical coherent optical communications system (see FIG. 1). In the standard derivation of the performance of shot-noise-limited heterodyne detection, it is usually assumed that the signal and local oscillator fields have constant amplitude and matched phase over the detector surface and that they have the same state of polarization. With these assumptions, it is easy to show that ##EQU1## where .eta. is the quantum efficiency of the detector, P.sub.S is the received optical signal power and W is the noise bandwidth of the IF filter. If the above assumptions are relaxed, the SNR is then given by H. A. Haus, Waves and Fields in Optoelectronics, Prentice-Hall, 1984; and R. H. Kingston, Detection of Optical and Infrared Radiation, Springer-Verlag, 1979) ##EQU2## where A is the area of the detector and E.sub.S and E.sub.LO are the complex field amplitudes of the signal and local oscillator waves, respectively. The SNR in (2) is simply the ideal SNR in (1) modified by m, which is referred to as the "mixing efficiency". This mixing efficiency is a measure of the match between the incoming signal and local oscillator fields and will have some slowly-varying, random value between 0 and 1.
For the special case where the complex field amplitudes E.sub.S and E.sub.LO are constant over the detector area and where the angle between the local oscillator and signal polarizations (assumed linear) is .theta., the SNR in (2) reduces to ##EQU3## The above expression for SNR is also valid for any state of polarization, if .theta. is appropriately defined. A derivation of the mixing efficiency for arbitrary states of polarization is given in Appendix A. For matched polarizations (.theta.=0.degree.), cos.sup.2 .theta.=1, and no sensitivity degradation is encountered. On the other hand, when the polarizations are orthogonal (.theta.=90.degree.), cos.sup.2 .theta.=0 and complete fading results, that is, no signal appears at the output of the IF filter. In general, the strength of the signal in the IF filter will vary slowly between these two extremes. It is therefore obvious that techniques must be employed to minimize or, if possible, eliminate this problem.
Several techniques have been proposed in the literature to handle the problem of polarization mismatch. These include the use of polarization-maintaining fibers, polarization-state controllers, polarization-diversity receivers (analogous to in-phase/quadrature radio receivers) and polarization-switching systems. Here we will concentrate on polarization-switching but, first, we will briefly review the non-polarization-switching methods for dealing with this problem (Section 2). Then (Section 3), several techniques based on polarization switching will be described. These schemes, which may be simpler to implement, all rely on forcing the polarization state of either the transmitted signal or the local oscillator to vary with time in a manner such that polarization-insensitive detection is possible. The result is a fixed level of detection performance, with a power penalty relative to ideal of 3 dB.